SCIENCE. 
65 
bottom of the containing vessel. There it will remain 
motionless until it dies ; but if it be again transferred to sea 
water it will recover, provided that its exposure to the fresh 
water has not been of too long duration. I have never 
known a naked-eyed Medusa survive an exposure of fifteen 
minutes : but they may survive an exposure of ten, and 
generally survive an exposure of five. But although they 
thus continue to live for an indefinite time, their vigor is 
conspicuously and permanently impaired. While in the 
fresh water irritability persists for a short time after spon- 
taneity has ceased, and the manubrium and tentacles are 
strongly retracted. 
Turning now to the case of the freshwater species, when 
first it is dropped into sea water at 85° there is no change in 
its movements for about fifteen seconds, although the ten- 
tacles may be retracted. But then, or a few seconds later, 
there generally occurs a series of two or three tonic spasms 
separated from one another by an interval of a few seconds. 
During the next half minute the ordinary contractions 
become progressively weaker, until they fade away into 
mere twitching convulsions, which affect different parts of 
the bell irregularly. After about a minute from the time of 
the first immersion all movement ceases, the bell remaining 
passive in partial systole. There is now no vestige of 
irritability. If transferred to fresh water after five minutes 
exposure, there immediately supervenes a strong and per- 
sistent tonic spasm, resembling rigor mortis, and the animal 
remains motionless for about twenty minutes. Slight 
twitching contractions then begin to display themselves, 
which, however, do not affect the whole bell, but occur 
partially. The tonic spasm continues progressively to 
increase in severity, and gives the outline of the margin a 
very irregular form ; the twitching contractions become 
weaker and less frequent, till at last they altogether die away. 
Irritability, however, still continues for a time — a nip with 
the forceps being followed by a bout of rhythmical contrac- 
tions. . Death occurs in several hours in strong and irregu- 
lar systole. 
If the exposure to sea wa*er has only lasted two minutes, 
a similar series of phenomena are presented, except that the 
spontaneous twitching movements supervene in much less 
time than twenty minutes. But an exposure of even one 
minute may determine a fatal result a few hours after the 
Medusa has been restored to fresh water. 
Contact with sea water causes an opalescence and essen- 
tial disintegration of the tissues, which precisely resemble 
the effects of fresh water upon the marine Medusae. When 
immersed in sea water this Medusa floats upon the surface, 
owing to its smaller specific gravity. 
In diluted sea water (50 per cent) the preliminary tonic 
spasms do not occur, but all the other phases are the same, 
though extended through a longer period. In sea water 
still more diluted (1 in 4 or 6) there is a gradual loss of 
spontaneity, till all movement ceases, shortly after which 
irritability also disappears ; manubrium and tentacles ex- 
panded. After an hour’s continued exposure intense rigor 
mortis slowly and progressively develops itself, so that at 
last the bell has shrivelled almost to nothing. An exposure 
of a few minutes to this .strength places the animal past 
recovery when restored to fresh water. In still weaker 
mixtures (1 in 8, or 1 in ten) spontaneity persists for a 
long time; but the animal gradually becomes less and less 
energetic, till at last it will only move in a bout of feeble 
pulsations when irritated. Instill weaker solutions (1 in 
12, or 1 in 15) spontaneity continues for hours, and in solu- 
tions of from 1 in 15 to 1 in 18 the Medusa will swim about 
for days. 
It will be seen from this account that the freshwater Me- 
dusa is even more intolerant of sea water than are the ma- 
rine species of freshwater. Moreover the freshwater Medusa 
is beyond all comparison more intolerant of sea water than 
are the marine species of brine. For I have previously 
found that the marine species will survive many hours’ im- 
mersion in a saturated solution of salt. While in such a 
solution they are motionless, with manubrium and tentacles 
relaxed, so resembling the freshwater Medusa shortly after 
being immersed in a mixture of 1 part sea water to 5 of 
fresh ; but there is the great difference that while this small 
amount of salt is very quickly fatal to the fresh-water 
species, the large addition of salt exerts no permanently 
deleterious influence on the marine species. 
We have thus altogether a curious set of cross relations. 
It would appear that a much less profound physiological 
change would be required to transmute a sea-water' jelly- 
fish into a jelly-fish adapted to inhabit brine, than would 
be required to enable it to inhabit fresh water. Yet the 
latter is the direction in which the modification has taken 
place, and taken place so completely that sea water is now 
more poisonous to the modified species than is fresh water 
to the unmodified. There can be no doubt that the modifi- 
cation was gradual — probably brought about by the ances- 
tors of the freshwater Medusa penetrating higher and higher 
through the brackish waters of estuaries into the fresh water 
of rivers — and it would I think be hard to point to a more 
remarkable case of profound physiological modification in 
adaptation to changed conditions of life. If an animal so 
exceedingly intolerant of fresh water as is a marine jelly- 
fish may yet have all its tissues changed so as to adapt them 
to thrive in fresh water, and even die after an exposure of 
one minute to their ancestral element, assuredly we can see 
no reason why anv animal in earth or sea or anywhere else 
may not in time become fitted to change its element. 
George J. Romanes. 
A NEW GENUS OF RHINOCERONTID^E. 
While the genus Aphelops must be regarded as the direct 
ancestor of the recent rhinoceroses with canine and incisor 
teeth, now confined to Asia and the Islands, the ancestral 
genus of the African forms and their extinct congeners, 
which are without the teeth named, is less known. It can 
now be shown that the missing genus inhabited North 
America, and that, like Aphelops , it is hornless. It may be 
named and characterized as follows : Peraceras , Cope ; supe- 
rior dentition; I. o ; C. o ; P-m. 4; M. 3 ; nasal bones 
weak, hornless. 
This genus is established on a new basis recently dis- 
covered by Mr. R. H. Hazard, in the Loup Fork formation 
of Nebraska, which may be called Peraceras superciliosus. 
It is founded on a nearly perfect skull, which lacks the 
lower jaw. Its size is about that of the Indian rhinoceros. 
It is narrowed anteriorly, but is very wide between the 
orbits. Posterior to these it contracts rapidly, and rises to 
a rather elevated occiput. Saggital crest narrow; a promi- 
nent angle above each orbit. The premaxillary bone is 
narrow and weak. The nasal notch extends to above the 
middle of the third superior premolar. The occiput is rec- 
tangular in outline, with truncate summit. Its surface 
above is concave, divided by a strong median crest ; lower 
down a vertical groove intersects its lateral border. The 
crests of the molar teeth are rather simple, and the poste- 
rior notch is soon isolated on attrition. Wear also isolates 
an external median fossa of the second premolar. Length 
of skuil from end of premaxillary bone to condyles, M. 700 ; 
length of alveolar border of premaxillary, .025 ; length of 
molar series, .315 ; length of three true molars, .160; width 
of crown of second true molar at base, .075 ; superciliary 
width, .255. 
This species is nearest to the Peraceras malacorhinus, a 
species which I foimerly referred to Aphelops , but which I 
have little doubt belongs to the present genus. It differs 
from P. superciliosus as follows : In the latter species the 
front is wider, and is plane or concave, not convex ; the 
superior edge of the maxillary is not wide and incurved, 
and has not the oblique ridges ; the infraorbital foramen 
consequently has a more lateral opening. The nareal notch 
does not extend so far p steriorly by the one and a half 
molar teeth. The occiput is wider, is divided by a median 
crest not found in P. malacorhinus, and has the vertical 
lateral grooves much shorter. The acute supraorbital angle 
is not seen in the P. malacorhinus. 
The rhinoceroses of the Loup Fork formation whose 
generic position can now be ascertained, are the following: 
Paraceras malacorhinus ; P. superciliosus ; Aphelops mericii- 
anus ; A. Negalodus ; A. fossiger . — Am. Naturalist. 
